1. Field of the Invention
The present invention relates to a valve operating device for an internal combustion engine equipped with a variable valve timing and valve-lift characteristic mechanism, and in particular being capable of changing both valve timing and valve-lift characteristic (lifted period and valve lift) of intake and/or exhaust valves depending on engine operating conditions.
2. Description of the Prior Art
In recent years, there have been proposed and developed various internal combustion engines equipped with a valve operating device enabling valve timing and valve-lift characteristic (lifted period and valve lift) to be varied depending on engine operating conditions, in order to reconcile both improved fuel economy during operation at low and middle engine speeds and enhanced engine output torque during operation at high engine speeds. One such valve operating device with variable valve timing and valve-lift characteristic mechanism has been disclosed in Japanese Patent Provisional Publication No. 7-279629. The valve operating device disclosed in the Japanese Patent Provisional Publication No. 7-279629 includes a camshaft driven by an engine crankshaft, a pair of low-speed and high-speed cams fixedly connected onto the camshaft in a manner so as to open two intake-port valves arranged in each individual engine cylinder, and first and second valve lifters slidably provided in respective lifter guide holes bored in an engine cylinder head for transmitting a cam lift of the selected one of the low-speed and high-speed cams to the intake-port valves. A substantially flat slider is also disposed between the low-speed/high-speed cam set and the first/second valve lifter set in such a manner as to be moveable in the axial direction of the camshaft. The slider has a plurality of cam followers on its upper surface. A switching means is provided to selectively switch the contact-position relationship between the cam followers and cams by virtue of the sliding movement of the above-mentioned slider, based on engine operating conditions. In detail, when the engine is operated in a low- or mid-speed range, the slider itself is slid in one axial direction of the camshaft so that the upper face of the first follower is brought into abutted-contact with the outer peripheral surface of the low-speed cam, and so that the first and second valve lifters are moved up and down together with the slider in accordance with the cam profile of the low-speed cam. This provides a comparatively small valve-lift characteristic (or a relatively small lifted period and valve lift) in the low- or middle-speed range. Conversely, when the engine is operated in a high-speed range, the slider is slid in the opposite axial direction of the camshaft so that the upper face of the first and second followers are brought into abutted-contact with the outer peripheral surface of the high-speed cam, and so that the first and second valve lifters are moved up and down together with the slider in accordance with the cam profile of the high-speed cam. This provides a comparatively large valve-lift characteristic (or a relatively large lifted period and valve lift). Thus, the conventional valve operating device can variably change valve timing as well as valve-lift characteristics (lifted period and valve lift) depending on engine operating conditions. Additionally, the conventional valve operating device as disclosed in the Japanese Patent Provisional Publication No. 7-279629 has various merits, for example, a compact and simple structure of the upside of the respective valve lifter, and enhanced layout flexibility in the engine room (owing to the valve operating device totally small-sized as a result of the use of the flat slider formed on its upper face with a plurality of followers).
In the conventional valve operating device which uses a flat slider with a plurality of follower portions to enable both valve timing and valve-lift characteristic to be varied, however, there is the following drawbacks.
Each of the follower portions with which the outer peripheral surface of each of the cams can be brought into abutted contact, is formed on the upper face of the slider in a manner so as to project from the slider upper face. Owing to a limited height of the valve system within a limited space in the internal combustion engine, as a matter of course, the projected amount of each of the follower portions is also limited. As a result of this, it is impossible to provide an adequate valvelift difference between a valve lift created by the low-speed cam and a valve lift created by the high-speed cam. This lowers a design flexibility of the engine. Due to the inadequate valve-lift difference, it is difficult to provide a satisfactory engine performance all over the engine operating range.
Accordingly, it is an object of the invention to provide a valve operating device for an internal combustion engine with a variable valve timing and valve-lift characteristic mechanism, which avoids the aforementioned disadvantages of the prior art.
In order to accomplish the aforementioned and other objects of the present invention, a valve operating device for an internal combustion engine having at least two engine valves per cylinder, the valve operating device enabling both valve timing and valve-lift characteristic to be varied depending on engine operating conditions, comprises a camshaft adapted to be driven by a crankshaft, at least one cam pair including a low-speed cam and a high-speed cam, each operating an associated valve of at least two engine valves included in a cylinder, and integrally formed on an outer periphery of the camshaft, a main rocker shaft supported on a cylinder head, a sub rocker shaft, at least one rocker arm set including a low-speed rocker arm having a first follower driven by the low-speed cam for operating the associated valve during a low-speed cam operating mode and oscillatingly supported by the main rocker shaft and mounting thereon the sub rocker shaft, and a high-speed rocker arm having a second follower driven by the high-speed cam for operating the associated valve during a high-speed cam operating mode and oscillatingly supported by the sub rocker shaft, the second follower of the high-speed rocker arm being closely juxtaposed to the first follower and located within a dead space defined in an outside of the at least two engine valves included in the engine cylinder, and a mode switching device provided for switching from one of the low-speed and high-speed cam operating modes to the other depending on the engine operating conditions, the mode switching device initiating the low-speed cam operating mode by disconnecting the low-speed rocker arm from the high-speed rocker arm, and initiating the high-speed cam operating mode by connecting the low-speed rocker arm to the high-speed rocker arm. It is preferable that the main rocker shaft comprises a plurality of divided rocker shaft members supported on the cylinder head and including relatively short endmost rocker shaft members respectively located closer to both ends of the engine and relatively long intermediate-divided rocker shaft members each being disposed between associated two cylinders adjoining to each other, and each of the plurality of divided rocker shaft members oscillatingly supports the low-speed rocker arm of the rocker arm set. More preferably, the at least one rocker arm set may comprise two adjacent rocker arm sets disposed between the associated two cylinders adjoining to each other, one of the two adjacent rocker arm sets has a symmetric shape with respect to the other, and the low-speed rocker arm included in the one rocker arm set and the low-speed rocker arm included in the other rocker arm set are oscillatingly supported on the same one of the relatively long intermediate divided rocker shaft members.